1. Field of the Invention
This invention relates bipolar-junction transistor (BJT)/MOSFET integrated circuit, particularly to resistive means to bias a BJT with a MOSFET.
2. Brief Description of Related Art
In integrated circuits, a polycrystalline film or diffusion is often used to fabricate resistors. Such resistors are limited in range of resistance values and may occupy substantial areas. Although it is well-known that a MOSFET can operate as a resistor in its ohmic or “triode” region of its drain V-I characteristics, it is seldom used in practice, because the resistance value varies with the gate voltage.
In circuit applications, a resistor is used to determine the current flow across a voltage differential, for instance between the supply voltage and an electrode of a transistor. For a BJT common emitter amplifier as shown in FIG. 1a, once the dc collector current IC1 is chosen, the dc base current IB must be 1/β times less. Normally, the value of beta varies over a wide range in any production line, and if IB is fed from a positive power supply through a resistor RB to the base of an NPN transistor, the resistance value must vary accordingly. Besides, the base resistance value may be too low to shunt the input signal Vin coupled to the base. If the collector is fed through an inductive load, there is no dc voltage drop the load and the minimum required VCC is the sum of dc base-to-emitter voltage VBE and the drop across the RB.
If the dc base current is supply from a current mirror as shown in FIG. 1b, the effective beta is reduced, and the input resistance is further reduced to shunt the input signal Vin.
In another instance as shown in FIG. 1c, it is desired to operate Q1 at a specified collector current IC1. Then the base current IB should be equal IC1/β. This base current can be derived from the reference base current of Q3 through current mirrors Q2,Q2′, where Q3 is fed from a current source with current mirror Q4, Q4′. A current mirror usually has a master section e.g. Q2′ and Q4′ and a slave section e.g. Q2 and Q4. The dc collector-to-emitter voltage of the master section with the base shorted to the collector must be equal to the turn-on voltage of the BJT, typically in the 0.7V-0.8 V range. The dc collector-to-emitter voltage of the slave section need not be as large, so long as the BJT operates in the active region typically >0.1-0.2V. The dc supply voltage must at least allow enough headroom VCE4 (<0.2V) for the current source Q4, the base-to-emitter voltage VBE3 (>0.7V) of the reference BJT Q3 and the base-to-emitter voltage VCE2′ (>0.7) of the current mirror Q2′ for a total of more than 1.5 V. For a standard battery, the rated voltage is 1.5 V new, and for a mercury cell, the voltage is only 1.3V new. For low voltage application, it is desirable to operate the circuit below 1.3V. Thus the circuit shown in FIG. 1c needs at least a supply voltage VCC higher than VCE4(>0.2V)+VBE3(>0.7V)+VCE2′(>0.7V)>1.6V, which is not suitable for single battery low voltage operation.